Install fzf from GitHub releases instead of apt

[dguido]

Problem

The Dockerfile installs fzf from Ubuntu 24.04 apt (fzf package = v0.44.1), then separately downloads the shell integration scripts from the fzf GitHub repo:

RUN apt-get install -y ... fzf ...

ARG FZF_VERSION=0.44.1
RUN mkdir -p /home/vscode/.fzf && \
  curl -fsSL "https://raw.githubusercontent.com/junegunn/fzf/${FZF_VERSION}/shell/key-bindings.zsh" -o /home/vscode/.fzf/key-bindings.zsh && \
  curl -fsSL "https://raw.githubusercontent.com/junegunn/fzf/${FZF_VERSION}/shell/completion.zsh" -o /home/vscode/.fzf/completion.zsh

This has two issues:

1. fzf 0.44.1 is old — the current release is 0.67.0 with significant improvements
2. Downloading shell scripts separately is fragile — fzf 0.48+ has built-in fzf --zsh that generates shell integration natively, making the separate downloads unnecessary

Proposed fix

Remove fzf from the apt install list and install the binary from GitHub releases:

ARG FZF_VERSION=0.67.0
RUN ARCH=$(dpkg --print-architecture) && \
  case "${ARCH}" in \
    amd64) FZF_ARCH="linux_amd64" ;; \
    arm64) FZF_ARCH="linux_arm64" ;; \
    *) echo "Unsupported architecture: ${ARCH}" && exit 1 ;; \
  esac && \
  curl -fsSL "https://github.com/junegunn/fzf/releases/download/v${FZF_VERSION}/fzf-${FZF_VERSION}-${FZF_ARCH}.tar.gz" | tar -xz -C /usr/local/bin

Then in .zshrc, replace the manual sourcing:

# Before
source ~/.fzf/key-bindings.zsh
source ~/.fzf/completion.zsh

# After
eval "$(fzf --zsh)"

This is already how the trailofbits/skills devcontainer-setup plugin does it (per @DarkaMaul's review feedback on trailofbits/skills#26).